Photovoltaic cells have developed according to two distinct methods. The initial operational cells employed a matrix of single crystal silicon appropriately doped to produce a planar p-n junction. An intrinsic electric field established at the p-n junction produces a voltage by directing photon produced holes and free electrons in opposite directions. Despite good conversion efficiencies and long-term reliability, widespread energy collection using single-crystal silicon cells is thwarted by the exceptionally high cost of single crystal silicon material.
A second approach to produce photovoltaic cells is by depositing thin photovoltaic semiconductor films on a supporting substrate. Material requirements are minimized and technologies can be proposed for mass production. The thin film structures can be designed according to doped homojunction technology such as that involving silicon films, or can employ heterojunction approaches such as those using CdTe or chalcopyrite materials.
Despite significant improvements in individual cell conversion efficiencies for both single crystal and thin film approaches, photovoltaic energy collection has been generally restricted to applications having low power requirements. One factor impeding development of bulk power systems is the problem of economically collecting the energy from an extensive collection surface. Photovoltaic cells can be described as high current, low voltage devices. Typically, individual cell voltage is less than one volt. The current component is a substantial characteristic of the power generated. Efficient energy collection from an expansive surface must minimize resistive losses associated with the high current characteristic. A way to minimize resistive losses is to reduce the size of individual cells and connect them in series. Thus, voltage is stepped through each cell while current and associated resistive losses are minimized.
It is readily recognized that making effective, durable series connections among multiple small cells can be laborious, difficult,and expensive. In order to approach economical mass production of series connected arrays of individual cells, a number of factors must be considered in addition to the type of photovoltaic materials chosen. These include the substrate employed and the process envisioned. Since thin films can be deposited over expansive areas, thin film technologies offer additional opportunities for mass production of interconnected arrays compared to inherently small, discrete single crystal silicon cells. Thus a number of U.S. Patents have issued proposing designs and processes to achieve series interconnections among the thin film photovoltaic cells. Many of these technologies comprise deposition of photovoltaic thin films on glass substrates followed by scribing to form smaller area individual cells. Multiple steps then follow to electrically connect the individual cells in series array. Examples of these proposed processes are presented in U.S. Pat. Nos. 4,443,651, 4,724,011, and 4,769,086 to Swartz, Turner et al, and Tanner et al., respectively. While expanding the opportunities for mass production of interconnected cell arrays compared with single crystal silicon approaches, glass substrates must inherently be processed on an individual batch basis.
More recently, developers have explored depositing wide area films using continuous roll-to-roll processing. This technology generally involves depositing thin films of photovoltaic material onto a continuously moving web. However, a challenge still remains regarding subdividing the expansive films into individual cells followed by interconnecting into a series connected array. U.S. Pat. No. 4,746,618 to Nath et al. teaches a design and process to achieve interconnected arrays using roll-to-roll processing of a web substrate such as thin stainless steel. The process includes multiple operations of cutting, selective deposition, and riveting. These operations, combined with the high cost of stainless steel sheet, add considerably to the final interconnected array cost. Similar concerns exist with the teachings of U.S. Pat. Nos. 4,965,655 to Grimmer et. al. and U.S. Pat. No. 4,697,041 to Okaniwa. These references teach processes requiring expensive laser scribing and interconnections achieved with laser heat staking. In addition, the two latter references teach a substrate of thin vacuum deposited metal back contact on films of relatively expensive polymers. The high electrical resistance and fragile nature of thin vacuum metallized layers significantly limits the permissible active area of the individual interconnected cells.
Therefore, there remains a need for improved structures and processes to permit economical production of large area interconnected arrays of photovoltaic cells.
In a somewhat removed segment of technology, a number of electrically conductive fillers have been used to produce electrically conductive thermoplastic materials. This technology generally involves mixing of the conductive filler into the thermoplastic prior to fabrication of the material into its final shape. Conductive fillers typically consist of high aspect ratio particles such as metal fibers, metal flakes, or highly structured carbon blacks, with the choice based on a number of cost/performance considerations.
Electrically conductive plastics have been used as bulk thermoplastic compositions or formulated into paints. Their development has been spurred in large part by electromagnetic radiation shielding and static discharge requirements for plastic components used in the electronics industry. Other known applications include resistive heating fibers and battery components.
In yet another separate technological segment, electroplating on plastic substrates has been employed to achieve decorative effects on items such as knobs, cosmetic closures, faucets, and automotive trim. ABS (acrylonitrile-butadiene-styrene) plastic dominates as the substrate of choice for most applications because of a blend of mechanical and process properties and ability to be uniformly etched. The overall plating process comprises many steps. First, the plastic substrate is chemically etched to microscopically roughen the surface. This is followed by depositing an initial metal layer by chemical reduction. This initial metal layer is normally copper or nickel of thickness typically one-half micrometer. The object is then electroplated with metals such as bright nickel and chromium to achieve the desired thickness and decorative effects. The process is very sensitive to processing variables used to fabricate the plastic substrate, limiting applications to carefully molded parts and designs. In addition, the many steps employing harsh chemicals make the process intrinsically costly and environmentally difficult. Finally, the sensitivity of ABS plastic to liquid hydrocarbons has prevented certain applications. The conventional technology for electroplating on plastic (etching, chemical reduction, electroplating) has been extensively documented and discussed in the public and commercial literature. See, for example, Saubestre, Transactions of the Institute of Metal Finishing, 1969, Vol. 47., or Arcilesi et al., Products Finishing, March 1984.
Many attempts have been made to simplify the process of electroplating on plastic substrates. Some involve special chemical techniques to produce an electrically conductive film on the surface. Typical examples of this approach are taught by U.S. Pat. No. 3,523,875 to Minklei, U.S. Pat. No. 3,682,786 to Brown et. al., and U.S. Pat. No. 3,619,382 to Lupinski. The electrically conductive film produced was then electroplated.
Another approach proposed to simplify electroplating of plastic substrates is incorporation of electrically conductive fillers into the resin to produce an electrically conductive plastic. The electrically conductive resin is then electroplated. Examples of this approach are the teachings of Adelman in U.S. Pat. No. 4,038,042 and Luch in U.S. Pat. No. 3,865,699. Adelman taught incorporation of conductive carbon black into a polymeric matrix to achieve electrical conductivity required for electroplating. The substrate was pre-etched to achieve adhesion of the subsequently electrodeposited metal. Luch taught incorporation of small amounts of sulfur into polymer-based compounds filled with conductive carbon black. The sulfur was shown to have two advantages. First, it participated in formation of a chemical bond between the polymer-based substrate and an initial Group VIII based metal electrodeposit. Second, the sulfur increased lateral growth of the Group VIII based metal electrodeposit over the surface of the substrate.
Since the polymer-based compositions taught by Luch could be electroplated directly without any pretreatments, they could be accurately defined as directly electroplateable resins (DER). Directly electroplateable resins, (DER), are characterized by the following features:
(a) having a polymer matrix; PA1 (b) presence of carbon black in amounts sufficient for the overall composition to have a an electrical volume resistivity of less than 1000 ohm-cm., e.g., 100 ohm-cm., 10 ohm-cm., 1 ohm-cm.; PA1 (c) presence of sulfur (including any sulfur provided by sulfur donors) in amounts greater than about 0.1% by weight of the overall polymer-carbon-sulfur composition; and PA1 (d) presence of the polymer, carbon, and sulfur in said directly electroplateable composition of matter in cooperative amounts required to achieve direct, uniform, rapid,and adherent coverage of said composition of matter with an electrodeposited Group VIII -based metal or Group VIII metal-based alloy.
The minimum workable level of carbon black required to achieve electrical resistivities less than 1000 ohm-cm. appears to be about 8 weight percent based on the weight of polymer plus carbon black.
Polymers such as polyvinyls, polyolefins, polystyrenes, elastomers, polyamides, and polyesters are suitable for a DER matrix, the choice generally being dictated by the physical properties required.
In order to eliminate ambiguity in terminology of the present specification and claims, the following definitions are supplied.
"Metal-based" refers to a substance having metallic properties and being composed of two or more elements of which at least one is an elemental metal.
"Polymer-based" refers to a substance composed, by volume, of 50 percent or more hydrocarbon polymer.
"Group VIII-based" refers to a metal (including alloys) containing, by weight, 50% to 100% metal from Group VIII of the Periodic Table of Elements.
It is important to note that electrical conductivity alone is insufficient to permit a plastic substrate to be directly electroplated. The plastic surface must be electrically conductive on a microscopic scale. For example, simply loading a small volume percentage of metal fibers may impart conductivity on a scale suitable for electromagnetic radiation shielding, but the fiber separation would likely prevent uniform direct electroplating. In addition, many conductive thermoplastic materials form a non-conductive surface skin during fabrication, effectively eliminating the surface conductivity required for direct electroplating.